Posted
by
CmdrTaco
on Tuesday September 14, 2010 @03:41PM
from the spent-all-day-playing-ctf dept.

coondoggie writes "NASA is looking hard at a way to blast spacecraft horizontally down an electrified track or gas-powered sled and into space, hitting speeds of about Mach 10. The craft would then return and land on a runway by the launch site."

new idea [imdb.com], exactly, but I guess it's good to see NASA looking at other possibilities. There are many. I remember MIT doing work on alternate launch technologies back in the seventies, if not earlier. The mass driver was one (a giant electromagnetic linear accelerator) although the idea was kicked around in science-fiction long before that. My current favorite is a possibly-reusable rocket whose reaction mass is water, using heat energy provided by ground-based lasers. You could launch things into orbit all day long with a setup like that. Probably need a dedicated nuclear power plant to run the thing.

Anyway, the other think to consider (especially for things like laser-based launches) is that the current "spit out a ton of speed really quickly and then coast your way to orbit" approach really sucks. Even a slow nice steady boost will get you to orbit without needing to hit escape velocity.

Who coasts into orbit? Once the engine cuts off in most any launch vehicle you've achieved orbit.

Get going at the right speed from the ground and you'll enter orbit as long as there's not a mountain in the way (you'd probably want to boost your periagee afterwards though). The main reason you go up before accelerating to orbital velocity is that you get above the atmosphere and don't lose as much energy.

Depends on what engine and what launch vehicle you are talking about actually. Some of the larger, multi-stage vehicles will cut off the main booster or first stage before achieving a true orbit. This is usually followed by such a short coast that the primary cut-off doesn't matter. The rest of the launch vehicle will continue gaining altitude until ignition of the second stage. This isn't always the case, but, like I said, it depends on the vehicle and payload you are flying.

Yes, Heinlein used this tech as a centerpiece enabling technology for Moon->Earth grain shipments (and as a kinetic weapon used against Earth once the rebellion started..."throwing rice") from a lunar penal colony in his superb science fiction novel "The Moon Is A Harsh Mistress". I highly recommend the story. Heinlein was amazing at predicting tech & science advances far, far ahead of any of his contemporaries.

In the above Heinlein novel, a rail launcher for Earth was proposed for several possible locations. These proposed locations shared certain characteristics, among them was elevation/altitude at the launcher exit point.

NASA could do a lot worse than taking some more inspiration (IIRC he's generally credited with the concept of communications satellites) from such an intellect.

"spit out a ton of speed really quickly and then coast your way to orbit" approach really sucks

A "nice slow steady boost" will burn an enormous amount of fuel.

Let's say your rocket weighs 1,000lb. If you provide = 1000lb of thrust your rocket will just sit there. If you provide 1001lb of thrust it'll start to accelerate every so slowly... if you provide 1002lb of thrust you'll accelerate twice as fast, but only burn ~0.1% more energy.

You'll go faster (for a given thrust) as you burn up fuel and thus shed weight, but at any weight, the higher the thrust, the smaller the percentage of energy you spend just overcoming gravity, and the more you spend accelerating the vehicle.

And don't forget, that if you got above the atmosphere "slow and steady"... if you're under orbit velocity, you're going to fall right down unless you plan on burning fuel forever.

There are a number of reasons why rail guns are more attractive than a "steady boost".First, we don't have anything that gives a steady boost for any reasonable amount of time at a reasonable amount of force. Rockets just don't last very long in the overall scheme of things, and laser-based propulsion systems don't have enough force to launch any appreciable payload (yet).Second, rail guns don't require you to accelerate fuel in order to keep on accelerating. This puts an effective limit on rockets, and anything the rail gun adds pushes out our capacity based on the fuel limit.Third, the higher/faster you're going before you start using conventional rockets will reduce fuel requirements, increase payload, or increase orbit. This is somewhat related to the second item, but not entirely. Conventional rockets require you to bring your fuel with you, which reduces payload capacity, and this compounds with the effects of being deeper in the gravity well.

And they never will. Lasers will NEVER be able to push anything into orbit, period. E=MC2. If you make E big enough to push a payload into orbit, your E ends up turning into M. Lasers only a bit more powerful than what we have now will end up creating matter in their pumping chambers and halting their output. The top few lasers on the planet are pretty close to the maximum power lasers can attain before spontaneously creating mass from the light they make. What may work, however is using a laser to be

There is no such thing as "coasting into orbit." When the power cuts off, you are either in orbit or you aren't. When there is no thrust, your kinematic state is determined for all time -- you're purely under the laws of freefall mechanics. If you're not in orbit when the engines cut, you'll either hit the ground or escape the gravity well. There's no transitional period -- if there were, what forces would be acting to cause the transition? Gravity is a conservative force.

As the GP said, gravity is a conservative force. [wikipedia.org] It is ALWAYS in control. Right now, gravity from distance stars is pulling us in their direction--the force is infinitestimal but present nonetheless. It is an extreme colloquialism to say that when you throw a ball up in the air gravity "takes control" when it starts to fall down, never mind that gravity caused the slowing of its ascent as well. Same as in orbits.

"Coasting into orbit," in your colloquial usage, simply means cutting the engines at a lower altitude than the final orbital altitude. To pull it off, you have to be going faster than orbital velocity at the lower altitude so that after your engine is cut off, some of your kinetic energy is transferred to potential energy, and you slow down while still going up until you reach the final orbit. This is no doubt used for small portions of most flights. But the GP's point is correct; anyone who understands Newtonian physics will be able to tell whether and what orbit you will reach once you cut your engines, thus no one ever bothers to talk about "coasting".

The only difference with a railgun-only launch system is you reach the maximum velocity at ground level and spend the *entire* trip to orbit "coasting." This is not what NASA is proposing. They will use the railgun only as the first stage, followed by scramjets and an orbital-insertion rocket engine, which is a much more realistic proposal.

Anyway, the other think to consider (especially for things like laser-based launches) is that the current "spit out a ton of speed really quickly and then coast your way to orbit" approach really sucks.

Why on earth was this moderated interesting? Is wrong information interesting now? You can't coast to orbit. When the power shuts off you either are in orbit or you aren't. Gravity doesn't take a holiday just because you are out of propellant.

Even a slow nice steady boost will get you to orbit without needing to hit escape velocity.

You can't get into orbit without hitting escape speed (escape velocity is actually a misleading term because it is a scalar). Escape speed doesn't have to be fast (in fact it can be any speed) but again, once the engines shut off you had better be at the escape sp

"Starr noted that electric tracks catapult rollercoaster riders daily at theme parks. But those tracks call for speeds of 60 mph -- enough to thrill riders, but not nearly fast enough to launch something into space. The launcher would need to reach at least 10 times that speed over the course of two miles in Starr's proposal."

Unfortunately escape velocity isn't Mach 10, but for early test platforms, we already have the tech necessary to do what's in the proposal, and what we might learn from repeated launches and fine tuning the

http://en.wikipedia.org/wiki/Scramjet#Theory [wikipedia.org]
Scramjets don't need supersonic airflow, they only need a dynamic pressure that is in the right interval. If you look at the equation in the wiki link you'll see that it will be able to operate at lower speeds at lower altitude, in fact, it will not be able to operate at mach 10 at low altitude at all, but would constantly speed up as it gains altitude to keep an approximately constant dynamic pressure.
Seems elegant enough for me.

im pretty sure mach 25 is orbital velocity. which would make mach 6 a bit over 20% of the speed needed. which is more than 6%.

Ouch, hope your physics teacher doesn't see your post. If E=1/2 * m * v**2, the ratio boils down to (mach6)**2 / (mach25)**2 which works out to 36/625 which works out to 5.76 percent. The energy required to gain altitude does matter, if you're going up a couple hundred miles. If it didn't, elevators wouldn't need motors. You are correct that the velocity is where most of the energy goes, and I didn't bother to verify the math, but its vaguely around the remaining 0.24 percent figure.

Yes, you've verified the staging equations, but you're still better off with a traditional, simple, ultra reliable first stage. The absolutely cheapest thing about orbital rocket launchers is the propellants, everything from R+D to launch support costs more. "Saving fuel" is a profoundly false economy for orbital launchers. A great way to spend billions to save millions (or less).

Also first stage is arguably the most phase of flight. A terrible place to "innovate".

There is great potential for energy savings, which would mean higher payloads and/or less expensive flights.

You want higher payloads, find a rocket that works, and make it bigger.

As for less expense, I take it you believe most of the cost of the space shuttle program is liquid H2? If so, you are horribly misinformed. If, by some utter miracle, the shuttle could be operated on flying unicorns instead of solid boosters and H2/O2, calculate the delta cost in the shuttle program. I think you'll be surprised how many decimal places you'll need to use.

Liquid H2 costs about a buck a pound in the quantities NASA uses. (We'd pay closer to two bucks a pound). The entire shuttle tank holds about a quarter million pounds of liq H2. No math phd required to figure that filling the fuel tank costs about a quarter mil. A similar level of math is required to multiply that by about 130 shuttle flights to get a lifetime program cost of a whopping 30 million or so. Wikipedia claims the total cost of the shuttle program from "I gotta idea" to end of program is about 175 Billion. So, liquid H2 fuel cost works out to 30 / 175000 * 100 = about 0.02 percent of total project cost. "Saving fuel" is simply irrelevant.

So, if we risk the lives of every crewman using a new non-man rated engine and/or delay the vehicle program by decades to develop and deploy the amazing fuel free flying unicorn engine system, we will save a whopping two hundredths of a percent of total program cost. Or rephrased, for the R+D to pay for itself, we need the total cost of R+D and deployment to remain below two hundredths of a percent of program cost.

Two hundredths of a percent of project cost is about what you budget for developing and deploying the HR diversity training, or perhaps company funded picnics. Not a realistic budget factor for a new primary propulsion system.

Maybe just missing a "0" -- should be "100 times", but even that's low. Mach 10 is around 6600 mph "where the jets go" and 7700 at sea level.

Of course, escape velocity is 25,000 mph (no friction from the air factored in), but (and I didn't read tfa) it seemed like they want to come back (maybe like a really big boomerang?), so I don't think it matters.

Just for grins, if the thing is launched at a 45 degree angle, it should reach a maximum height of approximately 185 miles, and travel a distance of around 75

According to TFA, the sled will be "hitting speeds of about Mach 10." That's fast, but then the TFA says, "electric tracks catapult rollercoaster riders daily at theme parks. But those tracks call for speeds of 60 mph -- enough to thrill riders, but not nearly fast enough to launch something into space. The launcher would need to reach at least 10 times that speed"

Sorry, but 10x roller coaster speeds isn't close to Mach 10.

NASA is on to something interesting here. It would seem that MagLev [wikipedia.org] is required (no wheels can handle that speed), and it would be interesting to see what kind of acceleration they can get out of LIM's [wikipedia.org]. Rocket propulsion seems a waste in this application. It might help bullet-train technology, and we can get some new spin-off inventions from NASA.

But when you catch the "track" to gain orbital velocity, wouldn't that just decay the track's orbit? You'd have to keep adding energy to the "track." I thought the rail gun concept was trying to avoid having to generate all that energy in space (thereby avoiding the need to launch all that extra weight)?

The force applied to the craft by the accelerator will also act against the accelerator. Firing the right way, it would drop the accelerator right out of orbit (it would impart it's velocity into the craft, leaving it with less than needed to maintain orbit, crashing down). Fired the other direction, and the exact reverse would be seen - the accelerator would "push" off of the craft, accelerating and gaining altitude, but the craft would then fall quite

Well that might defeat the purpose of such a system. The whole idea behind rail launching anything is to make launches cheaper and simpler. Having to use a rocket stage, to get to low orbit, to rendezvous with an orbital track, to propel a vehicle to its intended orbit probably doesn't match either of those criteria. Besides, the assumption that we would be launching from a track surrounded by atmosphere is pretty unimaginative. The way I see something like this working would involve building more of a tube

The problem with your idea is that the mouth of the tube should be open, otherwise how would the spacecraft come out? To be open it should need to be in a vacuum, otherwise air would come rushing in. You would need a tube extending all the way to above the atmosphere, let's say a hundred kilometers up.

to 'catch' the track as you say, the space craft must be going faster than it

No. All they need to do is to be at the same place at the same time.

The spacecraft is at the top of its trajectory, zero vertical speed, zero horizontal speed. Exactly at the same time the front of the track reaches that same location. A magnetic force catches the spacecraft so it won't fall down. The same magnetic force accelerates the spacecraft horizontally along the track so it will have orbital velocity by the time it reaches

You're forgetting that we could just slingshot around the sun. I've seen it in the movies and it works all of the time. After the slingshot maneuver, the velocity would be high enough to catch the track, which further accelerates the payload. The track probably has to then slingshot around the sun too, but these are just details to let the engineers figure out. Visionaries like me and the GP can then move on to better problems. Currently I'm working on a way to speed up my commute by slingshotting arou

According to TFA, the sled will be "hitting speeds of about Mach 10." That's fast, but then the TFA says, "electric tracks catapult rollercoaster riders daily at theme parks. But those tracks call for speeds of 60 mph -- enough to thrill riders, but not nearly fast enough to launch something into space. The launcher would need to reach at least 10 times that speed"

Sorry, but 10x roller coaster speeds isn't close to Mach 10.

I think he is looking for more like 128x. Furthermore TFA calls for reaching th

Accelerating up to supersonic speeds on a maglev track is quite problematic from a controls/stability perspective. The generated shock waves will bounce off of the ground/track creating some interesting ground effects which will mess up the launch unless properly controlled. I'm sure their proposal is to get the sled up to about Mach 1, at which point they'll be able to take off with a ramjet engine. Once they reach around Mach 5 in the atmosphere, they could transition into a scramjet configuration which c

LIMs are 'theoretically' unlimited, you just have to space them out properly and sequence them fast enough. In practice of course, its entirely different. I think for most practical purposes of terrestrial motion, they will be practically unlimited until we invent inertial dampeners of some sort.

Hey, it looks like someone read that Net [bussjaeger.org] Assets [smashwords.com] novel by one Carl Bussjaeger but decided that the trick could be done without using the libertarian sauce Bussjaeger pours over it. Bussjaeger ended up deciding that a rail gun or other tracked thing would not work so he went with a supersonic ground effect launcher.

After all the hype that we've been hearing over the years about rail-guns and seeing a few military and hobbyist demos on video sites, this one piece of near-former sci-fi may be finally coming to fruition as a usable approach. It's a great example of the sort of thing that had to wait for technological improvements and refinements, rather than a fundamental scientific or technological breakthrough, and is the convergence of several technologies. I'm encouraged to see more progress on such things which seems to have in recent years been eclipsed by information technology's faster cycles and overhyping in media (and I say this as someone who makes his living as a software engineer).

After all the hype that we've been hearing over the years about rail-guns and seeing a few military and hobbyist demos on video sites, this one piece of near-former sci-fi may be finally coming to fruition as a usable approach. It's a great example of the sort of thing that had to wait for technological improvements and refinements, rather than a fundamental scientific or technological breakthrough, and is the convergence of several technologies. I'm encouraged to see more progress on such things which seems to have in recent years been eclipsed by information technology's faster cycles and overhyping in media (and I say this as someone who makes his living as a software engineer).

I, well, I agree. And make my living the same way. I've also been a science-fiction fan since I was a kid (Clarke, Heinlein, Norton, Silverberg, Harrison, Dick, you name it I probably read it) and honestly I've been disappointed by the past forty years, at least so far as near-space development is concerned. I thought, well, I'd hoped we would be way further along than we are, and had we continued the pace of development after the end of the Apollo program we would have be. But we chickened out, let our lea

After all the hype that we've been hearing over the years about rail-guns and seeing a few military and hobbyist demos on video sites, this one piece of near-former sci-fi may be finally coming to fruition as a usable approach.

Nope, this piece of "near former sci-fi" is just as far from fruition as it ever was.

It's a great example of the sort of thing that had to wait for technological improvements and refinements, rather than a fundamental scientific or technological breakthrough, and is the conver

The basic problem with a railgun is that it give only a fraction of the velocity required - and it does so only in one plane.

Maybe I'm missing something, but can't you point the railgun in any direction you want? Granted it's probably cheaper to run the track along the ground, but you could at least in principle aim it straight up, or diagonally, or any other direction...

Incidentally, I suspect the appeal of the railgun is similar to the appeal of a Space Elevator... if you can supply the fuel/energy from

Well you could you know, make it one faster, you know go up to Mach eleven. Well, it's one faster, isn't it? It's not ten. You see, the Russians, you know, will be launching at Mach ten. You're on Mach ten here, all the way up, all the way up, all the way up, you're on Mach ten on your magnetic sled. Where can you go from there? Where? Nowhere. Exactly. What we do is, if we need that extra push over the cliff, you know what we do? Mach Eleven. Exactly. One faster.

built alongside the crawlerway? Just when I thought real estate prices in the Space Coast of Florida couldn't drop any lower, now we have sonic booms being generated at ground level just a couple miles from Merritt Island and Cocoa Beach.

So, the rail takes the x-43-like launcher to 600 (10x60) mph? That's not nearly enough to ignite the engine. Assuming it gets 5 times as fast (3000 mph should be enough to ignite it) it will be very close to the ground. 3000 mph close to the ground must generate non-trivial amounts of heat (and broken windows). Ignore that (because the launcher appears to have SR-71-like engines) for a moment and imagine the launcher now has to propel itself to the upper atmosphere, where it reaches Mach 10 (something we ne

fast enough to kill a human. I may be mistaken, but I am pretty sure that is the case. Current fighter pilots reach speeds high enough to black out (and/or red out). At said speed, they can't reach orbit. I can see no way for us to create a purely (or even mostly) land based launch system to supply enough energy in a short period of time to reach escape velocity. Not even if you built it on the Tibetan plateau that reaches 5000 meters above sea level.

You black out when you fly a curved path -- that's the only way to generate sufficient accelerations. To merely fly fast, all you've got is the mass of the aircraft (and your butt) counteracted by the engine thrust. Gives a nice buttkick, but not nearly enough to cause any distress.

The fighter pilots black out when they make turns, and for that they don't need to fly fast at all. You can easily black out on an aerobatic biplane with a prop engine.

Why would they do this at sea level? This should be done somewhere in the American West, at altitude. At 10K feet there is a heck of a lot less air resistance. Could be done on one of the Air Force ranges for sonic boom sake.

I was thinking this too. Edwards AFB maybe?
Hmmm... only about 2500 ft apparently. We can get
to 5000 feet easily on a lot of places out west though,
and I'm sure the military already controls a lot of flat
land at that altitude or greater.
10000? I don't think you can do that without the
extra hassle of building on very steep land.

I'm a complete layman here, but it seems to me that friction from air would be a serious problem at the speeds a vehicle would have to be propelled off this launcher. By the time conventional rockets have achieved a significant speed they're already fairly high in the atmosphere. I can't see a launch tower being practically ramped up high enough to overcome these effects. The vehicle would have to survive the stresses of heat and friction at launch and reentry. There's also the matter of drag kicking in bef

This sounds comparable to firing a bullet from a gun which seems like it would be a rather violent launch.

Depends. If you used a linear accelerator (e.g. a mass driver) of sufficient length, you could accelerate at one G. You'd probably want more than that, though, to keep the size of the launcher manageable.

It looks more like the rail truck accelerates the launch vehicle to mach 1 which leaves the end of the track and the scramjet lights and carries it and its payload to mach 10 at about 20 miles altitude. The payload then separates from the launcher, the rocket ignites and sends the payload into orbit. The launch vehicle returns and lands for reuse.

I agree that this idea has been around a while -- it's still a great idea.

Scramjets are really pretty simple devices compared to rocket engines. This machine would be like the first and second stages of a three-stage rocket, saving something like 80% of the mass. (OK, most of that mass is relatively cheap kerosene and LOX, but still.) Getting a sled up to Mach 1 to get the scramjets started is really not that challenging. If they don't start correctly, you just slow down... and nothing bad happens.

Everyone is banging their head over trying to hit Mach 10 on the track.

TFS and everyone else is misunderstanding the proposal.

The current idea is for the sled on the track to accelerate a scramjet up to about 600mph, then the scramjet lifts off, flies up to altitude and at about mach 10, releases a rocket which boosts the payload into orbit.

If I've learned one thing from Superman, it's that the formula for flight is up, up, and away. Therefor it is 2/3 up, and 1/3 away.

Yes, but I understand that for other countries the formula may be different. Remember, he was all about Truth, Justice and the American way. So, for example, when Russia wants to launch a spacecraft, they have to use Up, Up, and the Soviet way, which as we all know, is somewhat different.

This needs more +1 insightful as well as some +1 informative. This is exactly what would happen. In fact rocket launches already take some advantage of this fact. Going absolutely straight up would cause a whole world of hurt on the vertical frame and require extra fuel. Launching on an angle mitigates this. Basically you're traveling further to get out of the atmosphere but using less energy overall. If ground based launch facilities can get it to 600 m/h and then a scram jet can get it to escape velocity

...then by definition they're launching at a tangent to the earth's surface. This means that they'll have to punch through a lot more atmosphere than they would have to with a traditional perpendicular launch. I wonder how much they're really gaining with this strategy.

Since they are depending on a miracle occurring in engine technology, maybe they are expecting an equal miracle to occur in wing technology, to get a high mach number wing with a slow glider like lift to drag ratio so they can pull up at the end of the launch rail thing. Piling on that many pipe dreams, they may as well ask for a flying unicorn.